1. Field of the Invention
The present invention relates to a method for forming an end portion of a cylindrical workpiece, particularly the method for forming the end portion of the cylindrical workpiece by a spinning process, to form a part of deformed cross section with non-rotation symmetry on the end portion of the cylindrical workpiece, for example.
2. Description of the Related Art
Heretofore, a method for reducing an end portion of a metallic tubular member by a spinning process, to form a tapered portion and a small diameter cylindrical portion next to it has been known. Also, a method and an apparatus for forming a non co-axial small diameter cylindrical portion on an unprocessed portion of a cylindrical workpiece have been disclosed in prior patents such as Japanese Patent Nos. 2957153 and 2957154 (corresponding to U.S. Pat. No. 6,067,833). According to the spinning process, it has been known in the filed of casings for use in automobile exhaust system parts, for example, that a small diameter cylindrical portion having a circular cross section is formed on an end portion of the cylindrical workpiece integrally therewith, to be easily connected with a part to be connected. Furthermore, Japanese Patent Laid-open Publication Nos. 2001-286955 and 2007-014983 disclose a spinning process for reducing a diameter of an end portion of a tube having an oval cross section to provide a circular cross section, or a spinning process for reducing a diameter of an end portion of a pipe having a circular cross section to provide a non axially symmetrical shape such as an oval or polygon.
On the other hand, as for a method and apparatus for forming plate members, a method and an apparatus for forming the plate member into a hut shape of deformed cross section have been proposed in Japanese Patent No. 3292570 and International Publication No. WO2005/056210, and the similar process has been disclosed in Japanese Patent No. 3744390.
Recently, it has been required for exhaust parts of vehicles to be formed small in size, and in such a configuration as being capable of avoiding interference with other neighboring parts. In order to meet the requirement, the end portion of the metallic tubular member may have to be formed into the one having a non circular cross section, or formed with a recess at a portion to be possibly interfered with the neighboring parts, to provide a deformed cross section. In this case, it can be formed into the one having the non circular cross section according to the aforementioned Japanese Patent Laid-open Publication Nos. 2001-286955 and 2007-014983. However, as the roller is to be revolved along a revolutionary locus with rotation symmetric having a center, such as a circle, oval, elongated circle, or the like, it can not be formed into the one having a non-rotation symmetric cross section.
In contrast, according to the forming methods as described in Japanese Patent Nos. 3292570 and 3744390, and International Publication No. WO2005/056210, the plate member can be formed into the hut shape having the deformed cross section. However, those methods can not be applied to the cylindrical workpiece as they are. In addition, as they do not employ a sequential process, but they employ a single locus process, which is different from the spinning process as disclosed in the aforementioned prior publications, they can not be used for the spinning process applied to the end portion of the cylindrical workpiece.